Magnetorheological (MR) fluids in general comprise a dispersion of low coercivity, ferromagnetic particles in a liquid vehicle. In the absence of a significant magnetic field, the fluid has an off-state viscosity, and in the presence of a suitable magnetic field (on-state), the fluid gels and requires a substantial torque or yield stress to commence flow. The particles are often small iron particles while a broad range of nonmagnetic liquids have been used or proposed for MR fluids. In some applications lubricants are mixed with the liquid vehicle when lubrication properties are required. Additives may also be employed to maintain the suspension of the particles in the liquid and to prevent coagulation of the particles.
U.S. Pat. No. 5,667,715 to Foister and assigned to the assignee of this invention describes a family of magnetorheological (MR) fluids that are very useful because they display a high ratio of the torque output generated by the magnetically activated MR fluid to the torque or output for the same fluid in the unactivated or off state. It is usually desired that this ratio, called the "turn-up ratio", be maximized under given operating conditions because one usually wants the off-state viscosity to be of a relatively low value while a relatively high on-state (magnetic field on) yield stress is desired to accomplish the intended function of an MR device. The '715 patent MR materials provide high turn up ratio in part by utilizing magnetic particles of two distinct complementary sizes dispersed in a suitable liquid vehicle.
In the automotive field, MR fluids are being evaluated for use in shock absorbers and other suspension devices. MR fluids are also being evaluated for use in clutches such as cooling fan clutches and in powertrain transmissions. In MR suspension devices, the operating temperatures normally do not exceed about 150.degree. C., and only normal gravitation forces act on the fluid to induce separation of the suspended particles. Many of the compositions disclosed in the '715 patent are satisfactory for such relatively low temperature and low separation force applications. However, some clutch applications, particularly plate applications utilizing multiple flat plates involve centrifugal forces that cause particle separation. Further, some automotive underhood applications involve operating temperatures of 200.degree. C. to 250.degree. C. Heretofore there have been no MR formulations having suitable stability for such demanding applications.